V(D)J recombination is targeted by recombination signal sequences (RSS) which are found immediately adjacent to functional immune receptor gene segments. While the RSS flanking D/H segments appear to be equivalent they are not randomly utilized. We have found that murine D/H genes rearrange at least 1000 times more frequently by deletion than by inversion. In human B cells, deletion D/H/J/H rearrangement is favored by 10-100 fold over inversional rearrangement. Artificial recombination substrates demonstrate that the strong bias for the 3' D/H RSS in D/H-J/H rearrangement can be recapitulated in cell lines, and is dependent on several factors including the coding sequence immediately adjacent to the D/H RSS, the recombination partner to which the D/H segment itself. These data suggest that certain RSS combinations target recombination better than others. Inverted D/H segments encodes positively charged residues at approximately twice the rate as D/H segments in the normal orientation. Data forms several different investigators have demonstrated the following: 1) an association between anti-DNA specificity and positively charged heavy chain CDR3/s; and 2) a propensity to find inverted D/H gene segments in anti-DNA antibodies. Since inverted D/H rearrangements are relatively rare, the high frequency of D/H inversions in antibodies with specificity for DNA warrants further study. Thus, the objectives of this proposal are to delineate the cis and trans- acting elements which target specific 12/23 RSS combinations using the preferential joining of the 5'D/H RSS to V/H RSS and 3' D/H RSS to J/H. and to determine whether D/H RSS targeting is dysregulated in autoimmune conditions.